Method and apparatus for thermal development with supporting surface for a development medium

ABSTRACT

This invention relates to a method and apparatus for thermally developing a photosensitive element. The thermal development method includes heating the photosensitive element to a temperature sufficient to cause a portion of a composition layer in the element to liquefy, soften, or melt; supporting a development medium with a non-rotating surface to provide contact of the development medium with the heated photosensitive element; and providing relative movement between the development medium and the non-rotating surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to a method and apparatus for thermallydeveloping a photosensitive element, and particularly to a method andapparatus for supporting a development medium with a non-rotatingsurface to provide contact with the photosensitive element.

2. Description of Related Art

Flexographic printing plates are well known for use in printing surfaceswhich range from soft and easy to deform to relatively hard, such aspackaging materials, e.g., cardboard, plastic films, aluminum foils,etc. Flexographic printing plates can be prepared from photosensitiveelements containing photopolymerizable compositions, such as thosedescribed in U.S. Pat. Nos. 4,323,637 and 4,427,759. Thephotopolymerizable compositions generally comprise an elastomericbinder, at least one monomer and a photoinitiator. Photosensitiveelements generally have a photopolymerizable layer interposed between asupport and a coversheet or multilayer cover element. Upon imagewiseexposure to actinic radiation, photopolymerization of thephoto-polymerizable layer occurs in the exposed areas, thereby curingand rendering insoluble the exposed areas of the layer. Conventionally,the element is treated with a suitable solution, e.g., solvent oraqueous-based washout, to remove the unexposed areas of thephotopolymerizable layer leaving a printing relief which can be used forflexographic printing. However, developing systems that treat theelement with a solution are time consuming since drying for an extendedperiod (0.5 to 24 hours) is necessary to remove absorbed developersolution.

As an alternative to solution development, a “dry” thermal developmentprocess may be used which removes the unexposed areas without thesubsequent time-consuming drying step. In a thermal development process,the photosensitive layer, which has been imagewise exposed to actinicradiation, is contacted with an absorbent material at a temperaturesufficient to cause the composition in the unexposed portions of thephotosensitive layer to soften or melt and flow into an absorbentmaterial. See U.S. Pat. Nos. 3,060,023 (Burg et al.); 3,264,103 (Cohenet al.); 5,015,556 (Martens); 5,175,072 (Martens); 5,215,859 (Martens);and 5,279,697 (Peterson et al.). The exposed portions of thephotosensitive layer remain hard, that is do not soften or melt, at thesoftening temperature for the unexposed portions. The absorbent materialcollects the softened un-irradiated material and then isseparated/removed from the photosensitive layer. The cycle of heatingand contacting the photosensitive layer may need to be repeated severaltimes in order to sufficiently remove the flowable composition from theun-irradiated areas and form a relief structure suitable for printing.After such processing, there remains a raised relief structure ofirradiated, hardened composition that represents the irradiated image.

Processors for thermal development of flexographic printing elements areknown. U.S. Pat. No. 5,279,697 describes an automated process andapparatuses for handling an irradiated printing element andaccomplishing heating and pressing to remove the unirradiatedcomposition from the element. One embodiment of the thermal developmentapparatus shown in FIGS. 15 and 16 includes a heated plate that isbrought into intimate contact with an absorbent material which in turnis in contact with an upper surface of a flexible sheet that resides ona base. The heated plate remains in place for a sufficient time toliquify a portion of polymer material in the sheet and allow theliquified polymer material to be absorbed onto the absorbent material.The heated plate is removed and the flexible sheet and the absorbentmaterial advance together while rolls separate the absorbent materialfrom the formed flexographic sheet.

U.S. Pat. No. 5,279,697 describes another embodiment of an automatedprocess and apparatus for handling an irradiated printing element andaccomplishing repeated heating and pressing to remove the unirradiatedcomposition from the element. WO 2001/18604 also describes a method andapparatus for thermal processing a photosensitive element. In boththermal processing apparatuses the absorbent material is a continuoussheet of a web, typically a non-woven, which is passed over a hot roll.The hot roll is urged towards a drum carrying the photosensitive elementpressing the web against the photosensitive element and forming a nip.Heat is transferred by conduction from the hot roll, through theabsorbent web, to the photosensitive element upon contact so thetemperature of the composition layer is raised sufficiently to enablethe unirradiated portions of the composition layer to liquefy and beabsorbed into the absorbent web. As the drum and hot roll rotate incontact together, the web is pressed against the photosensitive elementto absorb the liquefied unirradiated composition and is then separatedfrom the element.

A problem sometimes arises in thermal development processors in whichthe rotating hot roller brings the absorbent material into contact withthe photosensitive element. Existing thermal development processes donot always adequately clean out or remove the uncured photopolymer fromthe recessed areas. In order to achieve improved relief uniformity thepressure at the nip between the hot roll carrying the absorbent materialand drum carrying the photosensitive element can be increased to impressthe absorbent material into the recessed areas. Increased nip pressurecompresses the element creating a wider contacting zone for the nip,i.e., footprint, on the element along an axial length of the drum aswell as increases the residence (i.e., dwell) time for the transfer ofheat to the element. However, increased residence time that raisestemperatures at or above the glass transition temperature of the basesupport for the element, can result in distortion of the resultingprinting form or plate. In addition the hot roll has a tendency todeflect or bow along its length, resulting in non-uniform application ofpressure to the photosensitive element along axial length of the contactzone. Non-uniform application of pressure can contribute to theresulting relief structure of the printing form having non-uniformrelief depth.

Relief printing forms having distortion/s in the support and/or thecured photopolymeric layer result in poor print performance. Inmulticolor printing, when one or more of the relief printing forms havedistortion the printed image has poor registration. Even in single colorprinting, distortion in the relief printing form may print an image thatis not an accurate reproduction of its original, so called imageinfidelity, by printing straight lines as curves for example. The reliefprinting form having distortion/s may also incompletely print the imagedue to intermittent contact of the inked surface of the printing form tothe printed substrate.

In addition, the existing thermal development processors can beexpensive and difficult to manufacture as well as maintain, due to themechanical complexity of the rotating and moveable hot roller and theneed to monitor temperature with sensors on the moving parts.

SUMMARY OF THE INVENTION

The present invention provides for an apparatus for forming a reliefpattern from a photosensitive element having an exterior surface andcontaining a composition layer capable of being partially liquefied. Theapparatus comprising means for heating the exterior surface to atemperature sufficient to cause a portion of the layer to liquefy; meansfor supporting a development medium to provide contact with the exteriorsurface adjacent the liquefied portion; and means for providing relativemovement between the development medium and the means for supporting;wherein the means for supporting comprises a non-rotating surfaceopposite the liquefied portion.

In accordance with another aspect of this invention there is provided amethod for forming a relief pattern from a photosensitive element havingan exterior surface and containing a composition layer capable of beingpartially liquefied. The method comprising heating the exterior surfaceto a temperature sufficient to cause a portion of the layer to liquefyand supporting a development medium for contacting with the exteriorsurface adjacent the liquefied portion; wherein the supporting stepcomprises contacting a non-rotating surface opposite the liquefiedportion, and providing relative movement between the development mediumand the non-rotating surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood from the following detaileddescription thereof in connection with the accompanying drawingdescribed as follows:

FIG. 1 is a schematic side view of a first embodiment of the presentinvention depicting a means for supporting a development medium toprovide contact with a photosensitive element, wherein the means forsupporting includes a non-rotating surface. The means for supporting hasa cross-sectional shape that is non-circular.

FIG. 2 is a schematic side view of a second embodiment of the presentinvention depicting the means for supporting the development medium tothe photosensitive element, wherein the means for supporting includes anon-rotating surface. The means for supporting has a cross-sectionalshape that is semi-circular.

FIG. 3 is a schematic side view of a third embodiment of the presentinvention depicting a non-rotating surface for the means for supportingthe development medium, and a means for removing the development mediumfrom the photosensitive element.

FIG. 4 is a schematic side view of a fourth embodiment of the presentinvention depicting the means for supporting a development mediumwherein the means for supporting includes a non-rotating surface and ameans for removing the development medium from the photosensitiveelement integral with the means for supporting.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Throughout the following detailed description, similar referencecharacters refer to similar elements in all figures of the drawings.

The present invention is an apparatus and process for thermallydeveloping a photosensitive element, preferably to form a flexographicprinting plate. The present invention contemplates an apparatus capableof heating a photosensitive element having a layer of compositioncapable of being partially liquefied to a temperature sufficient to meltor soften or liquefy at least a portion of the layer for any purpose. Inparticular, the present invention relates to an apparatus and processfor supporting a development medium during thermal development.

Thermal development heats the photosensitive element to a developmenttemperature that causes uncured portions of the composition layer toliquefy, i.e., melt or soften or flow, and be carried away by contactwith the development medium. The development medium may also be referredto herein as development material, absorbent material, absorbent web,and web. Cured portions of the photosensitive layer have a highermelting or softening or liquefying temperature than the uncured portionsand therefore do not melt, soften, or flow at the thermal developmenttemperatures. Thermal development of photosensitive elements to formflexographic printing plates is described in U.S. Pat. No. 5,015,556;U.S. Pat. No. 5,175,072; U.S. Pat. No. 5,215,859; and WO 98/13730. Thephotosensitive element includes a substrate and at least a compositionlayer mounted on the substrate. The composition layer is capable ofbeing partially liquefied.

The term “melt” is used to describe the behavior of the unirradiatedportions of the composition layer subjected to an elevated temperaturethat softens and reduces the viscosity to permit absorption by theabsorbent material. The material of the meltable portion of thecomposition layer is usually a viscoelastic material which does not havea sharp transition between a solid and a liquid, so the processfunctions to absorb the heated composition layer at any temperatureabove some threshold for absorption in the development medium. Thus, theunirradiated portions of the composition layer soften or liquefy whensubjected to an elevated temperature. However throughout thisspecification the terms “melting”, “softening”, and “liquefying” may beused to describe the behavior of the heated unirradiated portions of thecomposition layer, regardless of whether the composition may or may nothave a sharp transition temperature between a solid and a liquid state.A wide temperature range may be utilized to “melt” the composition layerfor the purposes of this invention. Absorption may be slower at lowertemperatures and faster at higher temperatures during successfuloperation of the process.

An apparatus suitable for thermally developing the photosensitiveelement is disclosed by U.S. Pat. No. 5,279,697, and also by Johnson etal. in U.S. Pat. No. 6,797,454 B1. The photosensitive element in allembodiments is in the form of a plate. The plate may be clamped onto adrum for thermal development in the round, or onto a flat base forthermal development in a press. However, it should be understood thatone of ordinary skill in the art could modify each of the disclosedapparatuses to accommodate the mounting of the photosensitive element inany form suitable for flexographic printing. The photosensitive elementcan include the form of a cylinder, i.e., a sleeve, or a plate-on-sleeveor plate-on-carrier. The photosensitive element may be a continuous,seamless or substantially seamless photopolymerizable composition layeradjacent a cylindrically-shaped support. The cylindrically-shapedsupport may be referred to as a sleeve. Typically, plate-on-sleeve is aphotosensitive element that includes at least the composition layer on aplanar support, which is then mounted onto a cylindrically-shapedsupport. Typically, the plate-on-carrier is a photosensitive elementthat includes at least the composition layer on a planar support, whichis then mounted onto a flexible sheet, known as a carrier sheet.Oftentimes, multiple photosensitive elements are mounted onto thecarrier at various spaced locations.

In the present invention, thermal development includes heating of anexterior surface 17 of the composition layer of the photosensitiveelement 16 to a temperature Tr sufficient to cause a portion of thelayer to liquefy. The thermal development process is conducted typicallywith more than one cycle of heating the element and contacting theelement with the development medium in order to remove the uncuredpolymer to a suitable relief depth because uncured portions of thecomposition layer may only partially liquefy upon heating. The at leastone photosensitive layer (and additional layer/s if present) can beheated by conduction, convection, radiation or other heating methods toa temperature sufficient to effect melting of the uncured portions butnot so high as to effect distortion of the cured portions of the layer.One or more additional layers disposed above the composition layer maysoften or melt or flow and be absorbed as well by a development medium.The development medium is a material absorbent to the uncured portionsof the composition layer. The development medium could also beconsidered a blotting material wherein the uncured portions are removedby the development medium. The photosensitive element is heated to asurface temperature above about 40° C. (104° F.), preferably from about40° C. to about 230° C. (104-446° F.) in order to effect melting orflowing of the uncured portions of the composition layer. The thermaltreating steps of heating the photosensitive element 16 and contactingan outermost surface of the element with the development medium can bedone at the same time, or in sequence provided that the uncured portionsof the photopolymerizable layer are still soft or in a melt state whencontacted with the development medium.

The present invention provides for a means for supporting thedevelopment medium with a support surface to provide contact of thedevelopment medium with the exterior surface of the photosensitiveelement. The means for supporting is disposed adjacent the exteriorsurface of the element, that is, adjacent the partially liquefiedportion of the composition layer. The means for supporting includes anon-rotating surface that supports the development medium on a side ofthe development medium opposite the partially liquefied portion. Themeans for supporting includes a support member having the non-rotatingsurface on which the development medium resides or traverses. A side ofthe development medium opposite the partially liquefied portion contactsat least a portion of the non-rotating surface of the means forsupporting.

The non-rotating surface of the means for supporting has advantages asit reduces the costs and simplifies the manufacture, operation, andmaintenance of the apparatus as well as the method of using theapparatus. Also, the non-rotating surface allows for a much greatervariety of shapes that can bring the development medium into contactwith the element at a nip. This is advantageous in that the shape of thenon-rotating surface can be selected to tailor the pressure distributionat the nip according to the needs of the system, i.e., element,apparatus, and method. The shape of the non-rotating surface could thusallow operation at lower total forces than are used in prior artapparatuses having a rotating roll. Additionally, the shape of thenon-rotating surface can be selected to manage the transfer of heat atthe nip (contact zone) independent of the pressure for contacting thedevelopment medium into the element. Thus, the printing form thatresults from thermal treatment with the non-rotating surface supportingthe development medium typically has improved clean out or removal ofthe uncured photopolymer from the recesses areas of the relief structureand improved uniformity of relief depth, while avoiding excessive heatat temperatures that tend to distort the form.

The surface of the means for supporting is non-rotating, that is, thesurface supporting the development medium has no or essentially no rateof motion relative to a fixed point in space during contact of thedevelopment medium to the photosensitive element. The non-rotatingsurface may be stationary or may move several degrees as a partialrevolution during contact of the development medium to thephotosensitive element. Preferably, the non-rotating surface supportingthe development medium does not revolve about an axis or a center formore than one revolution.

The invention includes embodiments, in which the means for supportinghas a circular cross-sectional shape, such as a roller for example, butthe circular means for supporting does not rotate or revolve that is,the non-rotating surface is stationary. The invention also includesembodiments in which the means for supporting has a circularcross-sectional shape, and the support surface of the means forsupporting moves several degrees. In this embodiment, the non-rotatingsupport surface moves to some extent but not so much as to complete arevolution during a cycle of heating and contacting the developmentmedium to the photosensitive element. The invention also includesembodiments in which the means for supporting has a non-circularcross-sectional shape, such as a wedge or ellipse for example, andprovides an arcuate or projecting edge for the non-rotating surface.Hereto, the non-rotating surface may be stationary or may move a severaldegrees in a partial revolution. In embodiments where the means forsupporting has a cross-sectional shape that is rectangular, it ispreferred that the photosensitive element reside on a base member thathas an arcuate outer surface, such as a drum or roller.

The means for supporting may move to position the non-rotating supportsurface adjacent to the exterior surface. A means for moving may movethe means for supporting to position the non-rotating surface forthermal development, and/or may move the means for supporting toposition the non-rotating surface into one or more orientations duringdevelopment. For example, a means for moving may engage the supportmember to move the support member toward and away from the exteriorsurface of the element. The moving means may move the support member todeliver the development medium into contact with the exterior surface ofthe element for development and to retract from contact when reliefstructure is formed in the element. Also, the support member may move toposition the non-rotating surface with the development medium toaccommodate the thickness of different photosensitive elements. Inaddition, the means for supporting may be moved prior to or duringcontact of the development medium to the exterior surface to positionthe non-rotating surface (with the development medium) in order toaccommodate clean-out of uncured photopolymerizable materials forvarious depths and types of relief areas. The means for moving may movethe support member in an arc (about a fixed point) or in a line toaccomplish desired orientation/s of the non-rotating surface forsuitable removal of partially liquefied portions of the compositionlayer. It is also contemplated that the means for moving may move themeans for supporting in an arc or in a line to one or more positionsalong or against the direction of travel of the exterior surface of thephotosensitive element. The means for moving may also be used to controlthe depth that the support member, i.e., the non-rotating surface,penetrates into the exterior surface of the element.

The means for supporting may be coupled to one or more actuatingdevices, such as air cylinders, as the means for moving the supportmember. In one embodiment, each end of the support member is mountedonto arms attached to a beam that moves the support member under theurging of one or more air cylinders. It is well within the skill ofthose in the art to contemplate other embodiments for the means formoving the support member. Pressure cylinders used as the means formoving can also function as a means for pressing the support member withthe development medium into the element. Alternatively the means formoving can function independent of the means for pressing. In anotherembodiment, the means for supporting is mounted to a frame or a subframeof the apparatus.

Preferably, the support member has a length at least as long as atransverse dimension of the photosensitive element on the base member,i.e., drum, and is aligned axially with the base member. It is alsocontemplated that the support member may have a length shorter than, andperhaps significantly shorter than, the transverse dimension of theelement on the base member. The support member having a length shorterthan the transverse dimension of the element may traverse the exteriorsurface of the photosensitive element as the base member rotates. Thatis, the support member would traverse in a spiral pattern on thephotosensitive element. The support member has a cross-sectional shapethat is not limited, and can include, for example, elliptical, arcuateincluding convex and concave surfaces, parabolic, circular,semi-circular, wedge, triangular, rectangular, and other polygonalshapes. The shape of the support member can be symmetrical orasymmetrical. The support member thus may have a cylindrical, asemi-cylindrical, or a non-cylindrical three-dimensional shape. Thesupport member may be solid or may include a cavity (not shown) toaccommodate sources of heating the support member or for circulating aheated fluid. The support member can also have the cavity to minimizeits weight and/or to minimize its thermal mass so as to have more rapidresponse to temperature changes or to minimize heat losses in undesiredlocations.

The material/s of constructing the support member is not limited, andinclude, but are not limited to, sheet metal, cast metal, machinedmetal, alloys, polymer composite materials, thermoplastic materials,thermoset materials, and combinations thereof. The material chosen forthe support member should be resistant to thermal distortion at thetemperatures associated with thermal processing, and able to resistdisplacement, i.e., bending and torsion, under the operating forces inthe process.

The non-rotating surface of the support member may optionally include acoating or may be otherwise modified to suit a desired purpose. Such as,for example, the non-rotating surface may be coated with a non-stick,low-friction material, such as a fluoropolymer, to improve transport ofthe development medium on the non-rotating surface. Another example isthat the non-rotating surface may be coated with a resilient material,such as an elastomer having a Shore A hardness between about 30 andabout 90, which may help to conform the support surface to thephotosensitive element and the forming relief structure. The coatingmaterial can be applied in any manner suitable for use. The non-rotatingsurface may also be modified by treating the surface, such as plating oranodizing, on a metal support member, or by means known to those skilledin the art to provide a polished or roughened surface.

The invention further includes means for providing relative movementbetween the development medium and the means for supporting. In mostembodiments, the photosensitive element and the development mediumshould move at the same or substantially the same linear speed toaccomplish thermal development. To maintain this level of relativemovement between the development medium and the exterior surface of thephotosensitive element, there has to be a substantial relative movementbetween the means for supporting and the development medium. Relativemovement may be provided by moving the development medium, or moving themeans for supporting with the non-rotating surface, or moving both thedevelopment medium and the means for supporting. In one embodiment, thedevelopment medium is a continuous web that traverses the non-rotatingsurface of the means for supporting that is in a fixed position relativeto the exterior surface of the element for thermal development. Inanother embodiment, the development medium is a continuous web thattraverses the non-rotating surface of the means for supporting that ismoved into one or more orientations by the means for moving duringthermal development.

In an embodiment of the thermal processor 10 shown in FIG. 1, the meansfor supporting 12 is disposed adjacent the exterior surface 17 of thephotosensitive element 16 that resides on a base member 18, i.e., drum.The means for supporting 12 includes a support member 20 having thenon-rotating surface 22 that supports the development medium 24. Theshape of the support member 20 is such that the non-rotating surface 22includes a radius of curvature sufficiently small to form a projectingedge 26 about which the development medium 24 traverses. The supportmember 20 has an elliptical shape wherein a narrowed end of the ellipseis the non-rotating surface 22 that supports the development medium 24in contact with the element 16. In this embodiment the projecting edge26 or narrow end directs the development medium 24 into a recess orrecesses of the relief forming areas along the width (transversedimension) of the element 16 and thus provides improved clean out of theuncured photopolymerizable material from the element. The radius ofcurvature is sufficiently small to achieve increased localized pressureat a nip 30 between the element 16 and the development medium 24.Embodiments in which the non-rotating surface 22 includes a projectingedge 26 achieves a nip contact footprint (or contact zone) that is muchless than the nip contact footprint formed by the rotating hot roll witha larger radius in the apparatuses of the prior art. The small nipcontact zone associated with this embodiment typically allows for lowerpressing forces to generate local unit contact pressures sufficient todrive the development medium into the relief-forming areas (compared toapparatuses of the prior art). The small contact zone at the nip 30 canalso minimize the duration (or width) of compression of thephotosensitive element 16, which may also help to control the transferof heat from the support member 20 (at the non-rotating surface) to theelement 16, and thus minimize distortion in the element due to excessheat.

Another advantage of a small contact zone formed by the non-rotatingsurface may be in reduction in the amount of vapor and/or condensategenerated by the thermal development process. Photosensitivecompositions may contain one or more components that can vaporize orvolatilize when the element is heated to the temperature or temperaturesnecessary for thermal development to occur. The components that canvaporize or volatilize are generally low molecular weight compoundsincluding monomer. The vapor can condense within a thermal developmentprocessor and drip uncontrolled onto different areas of the processorcreating a mess within the processor. The small contact zone reduces thecontact time of the heated surface with the development medium and thephotosensitive element, and thus may create less vapor and/orcondensate.

In embodiments where the support member 20 includes more than oneprojecting edges 26 which could be used as the non-rotating surface 22,such as in an ellipse or a wedge, it is contemplated that eachprojecting edge 26 have a radius of curvature that can be the same ordifferent. The availability of different radius of curvature for a givensupport member 20 can provide greater adaptability of the processor toaccommodate or improve clean-out of unpolymerized material fromdifferent relief structures being formed in photosensitive elements. Inthis case, the support member 20 may index to position the appropriateradius of curvature of the non-rotating surface 22 adjacent the exteriorsurface 17 of the photosensitive element 16, prior to thermaldevelopment of the element, or even between cycles of heating andcontacting the element. It is preferred that the support member 20 wouldnot index while contacting the photosensitive element 16.

In another embodiment of the apparatus 10 shown in FIG. 2, the shape ofthe support member 20 is semi-cylindrical having a cylindrical surfaceportion 34 that coincides with the non-rotating surface 22. Thecylindrical, non-rotating, surface 22 has a radius of curvature of lessthan about 40 millimeters, which is less than a radius of curvatureassociated with the hot roller in the prior art devices. The radius ofcurvature of this embodiment can be as small as the radius of curvaturedepicted in FIG. 1. Similar to the embodiment of FIG. 1, this embodimentalso achieves a nip contact footprint that is less than the nip contactfootprint formed by the rotating hot roll in the apparatuses of theprior art. In prior art apparatuses, the rotating hot roll hadrelatively large diameter (i.e., large radius of curvature) in order tominimize deflection of the roll along its longitudinal axis. (Otherwisea complex anti-deflection structure would be needed to backup a smallerdiameter rotating roll. Radial deflection of the roll is not desirablesince it causes non-uniform pressures at the nip.) Also, the small nipcontact zone associated with this embodiment allows for lower pressingforces sufficient to drive the development medium 24 into therelief-forming areas of the element 16 (compared to apparatuses of theprior art).

The support member 20 is adjacent a means for buttressing 35 thatreinforces or strengthens the support member 20, particularly when thesupport member is in pressing contact with the photosensitive element16. The means for buttressing 35 includes a buttress member 36. Themeans for buttressing 35 can optionally include a backup reinforcingstructure, either of which can be made relatively massive to minimize oreliminate deflection that may occur with the support member 20. It iswell within the skill of those in the art to determine suitable backupreinforcing structures to provide desired functionality. The buttressmember 36 may be mounted to the frame or a subframe of the apparatus 10and may be adapted with the means to move the support member 20, to thusmove the buttress member with the support member. As shown in FIG. 2,the support member 20 is mounted onto the buttress member 36.Alternatively, the support member 20 may be integral with the buttressmember 36. The presence of the non-rotating surface 22 in conjunctionwith the means for buttressing 35 allows for a smaller radius ofcurvature for the cylindrical surface portion 34 than would be possiblewith a rotating roll since this elegantly avoids or minimizes axialpressure non-uniformities due to deflection of the support member 20.

In embodiments where the support member 20 is mounted to a buttressmember 36, it is contemplated that the non-rotating surface 22 of thesupport member or the support member 20 itself can be removable from thebuttress member 36. By means well known to those skilled in the art, thenon-rotating surface 22 or the support member 20 can be coupled onto thebuttress member 36 such that the non-rotating surface 22 can be easilyreplaced for maintenance purposes, or in order to accommodate differentprocessing conditions requiring different nip contact footprint orrelief forming structures.

In embodiments where the non-rotating surface 22 includes the radius ofcurvature, the radius of curvature is less than 40 millimeters,preferably less than 35 millimeters, and most preferably 8 to 15millimeters.

The embodiment of the apparatus 10 shown in FIG. 3 is similar to theembodiment in FIG. 2 with the exception that a means for removing 40 thedevelopment medium 24 from the photosensitive element 16 is mounted tothe means for buttressing 35. The means for removing 40 forms a peelingsurface or edge 42 at which the development medium 24 can changedirection from a first direction 44 a where the development medium 24travels in contact with the photosensitive element 16 to a seconddirection 44 b where the development medium 24 is removed from theelement 16 and ultimately returns to a take up (not shown). In theembodiment shown the means for removing 40 is a blade-like member 45.This embodiment and alternative means for removing 40 the developmentmedium 24 from the photosensitive element 16 are described in co-filedpatent application (attorney docket number IM-1318).

The means for supporting 12 the development medium 24 provides contactof the medium 24 with the exterior surface 17 of the photosensitiveelement 16 at the nip 30. In this embodiment, the development medium 24remains in contact with the exterior surface 17 of the photosensitiveelement 16 beyond the nip 30 until reaching the means for removing 40.The development medium 24 and the photosensitive element 16 can remainin contact after the nip 30 for any distance to reach the means forremoving 40. It is desirable to have the point where the developmentmedium 24 is removed from the element 16 close to the nip 30 to minimizethe effects of the molten polymer cooling while in contact with thedevelopment medium. If the molten polymer cools too much while incontact with the development medium 24 it may be difficult to separatethe development medium from the element 16 and in some cases the mediummay even tear. However, locating the means for removing 40 to be closeto the nip 30 is limited by the structure of means for supporting 12and/or the means for removing 40 structure that can fit in the spaceafter the nip 30 and materials that can resist bending and torsion. Inone embodiment the distance from an exit of the nip 30 to the means forremoving 40 where the development medium 24 is peeled from the element16 is generally greater than 0.5 inch (1.27 cm). In another embodimentthe distance from the nip 30 to the means for removing 40 where thedevelopment medium 24 is peeled from the element 16 is 2 to 4 inch (5.1to 10.2 cm), and preferably about 3 inch (7.6 cm). The means forremoving 40 is located adjacent to the means for supporting 12 andadjacent the exterior surface 17 of the photosensitive element 16. Themeans for removing 40 can be part of but located away, or separate from,the means for supporting 12. The removing means 40 peels or removes thedevelopment medium 24 from the element 16 along an entire orsubstantially entire transverse dimension (i.e., width) of the element.

The means for removing 40 establishes a consistent location adjacent theexterior surface 17 of the photosensitive element 16 at which thedevelopment medium 24 separates from the element. The means for removing40 can aid in keeping the photosensitive element 16 held to an outersurface of the base member 18 (drum) during web separation, eliminatingor at least reducing lifting and sagging experienced by photosensitiveelements in thermal processors of the prior art. While so doing thestrains induced by the separating web on the photosensitive element 16while still hot are minimized or substantially eliminated.

The means for removing 40 can be any shape, provided that the shapeincludes a peeling surface or edge 42 at which the development medium 24can change direction from the first direction 44 a where the mediumtravels in contact with the photosensitive element 16 to the seconddirection 44 b where the medium ultimately returns to take up. Examplesof shapes for the means for removing 40 include, but are not limited to,a blade, a roller, a roller backed by one or more support rollers, anelliptical-shaped member, a wedge and combinations thereof. The meansfor removing 40 can be formed from any material suitable for useincluding, but not limited to, sheet metal, cast metal, machined metal,alloys, polymer composite materials, thermoplastic materials, thermosetmaterials, and combinations thereof. The material chosen for the meansfor removing 40 should be resistant to thermal distortion at thetemperatures associated with thermal processing, and able to resistdisplacement, i.e., bending and torsion, under the operating tensionforces (of the web) in the process.

The removing means 40 can be mounted to the frame or subframe or to anymember within the apparatus to in order accomplish its intendedfunction. In the embodiment shown in FIG. 3, the blade-like member 45 ofthe removing means 40 is mounted to the buttress member 36.Alternatively, the removing means 40 can be mounted onto a bracket thatis secured at each end in the frame of the processor 10. In oneembodiment, the means for removing 40 is held in a fixed positionrelative to the outer surface of a base member 18 supporting thephotosensitive element 16. In another embodiment, the means for removing40 is held in a fixed position relative to the exterior surface 17 ofthe photosensitive element 16. Either of these previous embodiments maybe particularly useful in thermal processors that develop photosensitiveelements of one thickness or a narrow range of thickness. In anotherembodiment, the means for removing 40 (or the bracket) is mounted suchthat the means for removing can move rotationally about a pivot point asthe development medium 24 is peeled from the photosensitive element 16.In this embodiment the removing means 40 can automatically adjust tophotosensitive elements 16 of different thickness that may be developedin the thermal processor 10.

In the embodiment of the apparatus 10 shown in FIG. 4, thecross-sectional shape of the support member 20 is substantiallyrectangular and has as the non-rotating surface 22 a flat orsubstantially planar surface 48 supporting the development medium 24.The support member 20 is mounted to the buttress member 36, but can beintegral with the buttress member 36. It is also contemplated that thenon-rotating surface 22 of the rectangular support member 20 can be aconcave surface which conforms or substantially conforms to a portion ofthe exterior surface of the photosensitive element 16 while in positionon the base member 18. The advantage mentioned above for small radius ofcurvature represented by the embodiments of FIGS. 1 and 2 withstanding,this embodiment of the non-rotating surface 22 provides a larger contactnip zone (than in FIGS. 1 and 2) which can increase heat transfer fromthe non-rotating surface 22 to the element 16 and so may allow thenon-rotating surface 22 to be heated to a lower operating temperature.

In this embodiment, the means for removing 40 the development medium 24from the element 16 is included with, but located away from, the meansfor supporting 12 the development medium 24. That is the means forsupporting 12 and the means for removing 40 are a single (monolithic)structure, unlike the embodiment of FIG. 3 where the means forsupporting 12 and the means for removing 40 are separate structures. Themeans for removing 40 is an extended member 49 of a surface of thesupport member 20 forming a peeling surface or edge 42 at which thedevelopment medium 24 can change direction from the first direction 44 awhere the development medium 24 travels in contact with the element 16to the second direction 44 b where the medium is removed from theelement. It should be understood that the single structure which hasseparated the function of the means for supporting 12 and the means forremoving 40, can be composite where each means is affixed to the supportmember 20, or, as is shown in FIG. 4, can be an integral structure. Thenon-rotating surface 22 and the means for removing 40 are included onthe support member 20, positioned adjacent the base member 18, i.e.,drum. The support member 20 or the buttress member 36 can be mounted toprovide relative movement between the base member 18 and the means forsupporting 12.

In one embodiment the processor 10 includes a means for heating (notshown) the non-rotating surface 22. The non-rotating surface 22 whenheated can maintain or further elevate the temperature of the exteriorsurface 17 of the composition layer to temperature T2. The means forheating the non-rotating surface 22 is not limited and can be by anymethod, including an electrical core heater, steam, oil, hot air, andother heating sources that can provide a temperature of the non-rotatingsurface 22 sufficient to melt a portion of the composition layer throughthe development medium 24. The support member 20 carrying thedevelopment medium 24 on the non-rotating surface 22 applies heat to theexterior surface 17 of the element 16 upon contact, raising thetemperature of the element and causing the uncured portions of thecomposition layer of the element to melt, soften, or flow into thedevelopment medium. The means for heating can be incorporated into oneor more locations including, at the non-rotating surface 22, in thesupport member 20, in the cavity of the support member 20, in thebuttress member 36, and/or in a cavity in the buttress member 36. Themeans for heating provides heat to at least the non-rotating surface inorder to supply some or all of the heat necessary to increase thetemperature of the exterior surface 17 of the element 16 to atemperature Tr sufficient to cause a portion of the layer to liquefy. Itmay also be desirable to heat the entire support member 20, some portionof the support member 20 and/or buttress member 36. Also, in embodimentswhere the support member 20 is mounted to a buttress member 36, thesupport member may be thermally isolated from a buttress member by meansof insulation. Alternatively, the support member 20 may be thermallycoupled to some or all of the mass of the buttress member 36 to increasethe thermal mass of the means for heating.

The apparatus further includes a means for supporting 50 thephotosensitive element 16. The means for supporting 50 thephotosensitive element 16 is not limited, and can include for example adrum, multiple rolls, and a planar support. In the embodiments shown themeans for supporting 50 the photosensitive element 16 is a drum actingas a base member 18, which has an outer surface for supporting theelement. The drum 18 is mounted for rotation on the frame of theprocessor 10 and rotates in a clockwise direction. The outer surface ofthe base member 18 may include one or more layers to provide additionalfunctionality to the outer surface, such as resiliency, tackiness,protection, etc. A resilient outer surface on the base member 18 canresult in a longer nip zone and can accommodate some minor misalignmentbetween the drum and the support member 20 for the development medium24.

The base member 18 may be equipped with a heater, which is provided tokeep the photosensitive element 16 at a stable starting temperatureindependent of the surrounding environment. Any means of heating thebase member 18 is acceptable, as long as the power capacity of theheater is sufficient to maintain a fairly constant selected skintemperature on the outer surface of the drum of about 50-150° F.(10-65.6° C.), preferably 70 to 95° F. (21.1-35° C.). The means forheating the drum 18 is capable of heating the drum to a temperaturecapable of heating the exterior surface 17 of the composition layer to atemperature T3. The heater may be an electrical heating blanket, such asa wire wound blanket. If the normal operating environment is carefullycontrolled to be at a constant temperature, the heater can be turned offor omitted from the apparatus. As is disclosed in WO 2001/18604, it isalso possible that the drum be cooled by cooling means, such as, ablower directing a stream of air at the surface of the photosensitiveelement and the drum and/or by the circulating of cooling fluid beneaththe surface of the drum to cool the support side of the element.

The processor 10 may include another heating means (not shown) locatedadjacent the base member 18. This heating means can be a focused radiantheater directed at an exterior surface 17 of the photosensitive element16 on the base member 18. The heater applies heat to the exteriorsurface 17 of the composition layer, elevating the temperature of theexterior surface 17 of the composition layer to a temperature T1. In oneembodiment, the heater elevates the temperature of the surface 17 of thecomposition layer to a temperature Tr sufficient to melt theunirradiated portion of the composition layer, causing a portion of thelayer to liquefy. In another embodiment, the radiant heater provides aportion of the heat necessary to raise the temperature of the surface 17to a temperature Tr sufficient to melt the unirradiated portion, causinga portion of the layer to liquefy. The heater can include a one or aplurality of tubular infrared heating bulb/s mounted in end supportsthat also provide electrical connections to the bulb/s. The heater canalso include a reflector adjacent to the bulb/s that acts to focus anddirect the infrared radiation toward the exterior surface 17 of theelement 16.

Temperature sensors may be mounted throughout the processor 10 tomonitor the temperature for the purpose of controlling the heatingelements in the base member 18, the means for supporting 12 thedevelopment medium 24, and the optional radiant heater.

The radiant heater acting as a first heating means, the means forheating the means for supporting the development medium, and the basemember heater acting as a third heating means, independently or in anycombination, are capable of heating the exterior surface 17 of thephotosensitive element 16 to a temperature sufficient to cause aportion, i.e., an unirradiated portion, of the composition layer toliquefy at Tr. The first heating means, the second heating means, andthe third heating means independently or in any combination constitute aheating station. A preferred heating station includes the first heatingmeans and the second heating means.

The apparatus 10 includes the means for supporting the developmentmedium 24 to the exterior surface 17 of the photosensitive element 16 toform the nip 30 between the non-rotating surface 22 and the base member18. In one embodiment the development medium 24 is a continuous web thatis unwound from a supply roll (not shown), traverses the non-rotatingsurface 22 at the nip 30 and the optional means for removing 40, andthen wound up on a take up roll (not shown). The web may pass over oneor more additional rolls from the supply roll to the support means 12,and may pass over one or more additional rolls from the support means tothe take up roll. The support member 20, supply roll, take-up roll, andthe one or more additional rolls can be mounted for rotation on theframe of the processor 10 or on a carriage that is movable so that itcan be rolled out of the frame of the processor when required forservicing. One or more of the additional rolls may guide, idle, and/ordrive the web through the processor 10.

The web of the development medium 24 may be under tension control,velocity control, or a combination thereof for transport through theprocessor 10. In one embodiment, especially suited for use with themeans for removing 40, the processor 10 includes means for applyingtension (not shown) to the development medium 24. The means for applyingtension is located away from the exterior surface 17 of the element 16,and downstream of the means for removing 40. The web in its path fromthe removing means 40 to the take-up roll transports about a drive rollhaving an abrasive outer surface to prevent slippage of the web. Atorque motor provides constant torque to the drive roll so as to apply aconstant or substantially constant tension to the development mediumweb, at least during removal of the web from the element 16. The webcontacts the abrasive outer surface of the drive roll and can loop overone or more idler rolls to aid in providing a substantially uniformtension to the web after the nip. The tension required to remove the webfrom the element 16 may change within a cycle, or from one cycle to anext cycle, during thermal development for a given photosensitiveelement. As such, a controller (not shown) for the torque motor canadjust the torque so that the tension in the web is accordingly changed.Other embodiments implementing constant or substantially constanttension of the web after the means for removing 40, can be contemplatedby those skilled in the art. A suitable range of the tension in the web24 for the tensioning means is from about 0.3 to 2 lbs/in (0.5 to 3.5Newtons/cm) in one embodiment, and in another embodiment 0.5 to 1.0lbs/in (0.875 to 1.75 Newtons/cm). In an alternate embodiment, theprocessor may include a motor to pull the web through the processor 10at a linear speed that is synchronized with a linear speed of the outersurfaces of the sheet 16 on the drum.

The present invention may also include a means for providing relativemotion between the base member 18 and the non-rotating surface 22 of themeans for supporting 12, so that the element 16 and the developmentmedium 24 can be brought into contact with the other. The means forproviding relative motion can include the means for moving the supportmember 20 for the development medium 24 as described above. Means forproviding relative movement can also be accomplished, for example, bymounting the means for supporting 50 the photosensitive element 16 ontoa bracket that is supported on the frame and actuating the bracket tomove the base member 18 toward the development medium. The means forproviding relative motion bring the means for supporting 12 thedevelopment medium 24 and the means for supporting 50 the element 16towards each other, the nip 30 is formed between the element 16 and thesupport member 20 with the development medium 24 between the element 16and the non-rotating surface 22. The nip 30 is the location where thenon-rotating surface 22 is in an engaged position against the basemember 18. The non-rotating surface 22 carrying the development medium24 is engaged against the element 16 in pressure contact. Providing suchmeans for relative motion is described in U.S. Pat. No. 5,279,697(Peterson et al.) and U.S. Pat. No. 6,797,454 B1 (Johnson et al.).

It is desirable to apply a uniform or substantially uniform pressure atthe nip 30 across the width of the element 16 during processing. Thisuniform pressure assumes that the image across the nip 20 is uniform;those skilled in the art will recognize that the pressure applied willvary locally as image elements pass through the nip. Pressure is appliedto force the absorbent web 24 into intimate contact with thephotosensitive element 16. Pressure between about 0.70 kilograms persquare centimeter and about 24 kilograms per square centimeter,preferably between about 2 kilograms per square centimeter and about 12kilograms per square centimeter in the nip area is adequate to enhancethe absorption from the element surface to the absorbent web withoutdistorting the composite photosensitive element.

The operation of the process for thermally developing the photosensitiveelement 16 begins with placing the element on to the drum 18. The drumheater or the radiant heater may be used to preheat the drum 18. Aheater (not shown) for the means for supporting 12 the developmentmedium 24 preheats the non-rotating surface 22 of the support member 20.The drum 18 starts turning and carries the element 16 with it. Theradiant heater may preheat the bulbs before the element 16 reaches theheater, and then switch to an operating setting to achieve the desiredtemperature for melting the composition layer on element 16. As aleading edge of the element 16 reaches the position where the web of thedevelopment medium 24 being carried by the non-rotating surface 22 willcontact the drum 18, the supporting means 12 moves to bring the web 24against the element 16. The photosensitive element 16 composition layeris heated to between 40 and 230° C. (104-392° F.) while in contact withthe development medium 24. The development medium 24 contacts theexterior surface 17 of the composition layer of the heatedphotosensitive element 16, and absorbs the liquefied portions of theelastomeric polymer from the unirradiated portions of the compositionlayer, forming a flexographic printing form in which portions areremoved to form a relief pattern or surface. By maintaining more or lessintimate contact of the development medium 24 with the composition layerthat is molten in the uncured regions, a transfer of the uncuredphotosensitive material from the photopolymerizable layer, i.e.,partially liquefied portions, to the development medium takes place.Intimate contact of the development medium 24 to the photopolymerizablelayer may be maintained by the pressing the layer and the developmentmedium together. The development medium 24 may be removed immediatelyafter traversing the nip 30 area. Optionally while still in the heatedcondition and at a location away from where the web 24 contacted theelement 16, the development medium 24 traverses a peeling edge 42 thatcauses the web to change direction 44 a from traveling with the elementto a substantially opposite direction 44 b, thereby removing thedevelopment medium 24 from the exterior surface 17 of the element 16 toreveal the relief structure. As a trailing edge of the element 16 passesthe nip 30, the radiant heater may cool down or turn off, the supportingmeans 12 can retract away from the nip 30 and the web may be stopped.The support member 18 may return the leading edge of the element 16 tothe start position to begin another cycle of heating the element,contacting the web to the element, and removing the web from theelement. A cycle of the steps of heating the photopolymerizable layer,contacting the molten (portions) layer with the development medium, andremoving the development medium can be repeated as many times asnecessary to adequately remove the uncured material from the compositionlayer and create sufficient relief depth. However, it is desirable tominimize the number of cycles for suitable system performance, andtypically the photopolymerizable element is thermally treated for 5 to15 cycles.

The method and apparatus of the present invention uses a non-rotatingsurface to support the development medium to reduce and/or eliminate theoccurrence of distortions or waves in the resulting relief printingelement. Another advantage is that the non-rotating surface providesimproved relief uniformity for the resulting element. Also themanufacture and maintenance of thermal development processors accordingto the present invention is simplified and reduced cost.

Photosensitive Element

The present invention is not limited to the type of element that isthermally processed. In one embodiment, the photosensitive element 16includes a flexible substrate and a composition layer mounted on thesubstrate. The composition layer is at least one layer on the substratecapable of being partially liquefied. Preferably, the photosensitiveelement 16 is an elastomeric printing element suitable for use as aflexographic printing form. The at least one layer on the substrate ispreferably a photosensitive layer, and most preferably aphotopolymerizable layer of an elastomeric composition wherein thephotosensitive layer can be selectively cured by actinic radiation. Asused herein, the term “photopolymerizable” encompasses systems that arephotopolymerizable, photocrosslinkable, or both. In cases where thecomposition layer comprises more than one photosensitive layer on theflexible substrate, the composition of each of the photosensitive layerscan be the same or different from any of the other photosensitivelayers.

The layer of the photosensitive composition is capable of partiallyliquefying upon thermal development. That is, during thermal developmentthe uncured composition must soften or melt at a reasonable processingor developing temperature. At least the exterior surface of thecomposition layer is heated to a temperature Tr sufficient to cause aportion of the layer to liquefy, soften or melt.

The photosensitive layer includes at least one monomer and aphotoinitiator, and optionally a binder. The at least one monomer is anaddition-polymerizable ethylenically unsaturated compound with at leastone terminal ethylenic group. Monomers that can be used in thephotosensitive layer are well known in the art and includemonofunctional acrylates and methacrylates, multifunctional acrylatesand methacrylates, and polyacryloyl oligomers. Further examples ofmonomers can be found in U.S. Pat. Nos. 4,323,636; 4,753,865; and4,726,877. A mixture of monomers may be used.

The photoinitiator is a compound that generates free radicals uponexposure to actinic radiation. Any of the known classes ofphotoinitiators, particularly free radical photoinitiators may be used.Alternatively, the photoinitiator may be a mixture of compounds, one ofwhich provides the free radicals when caused to do so by a sensitizeractivated by radiation.

The optional binder is a preformed polymer that serves as a matrix forthe monomer and photoinitiator prior to exposure and is a contributor tothe physical properties of the photopolymer both before and afterexposure. In one embodiment the optional binder is elastomeric. Anon-limiting example of an elastomeric binder is an A-B-A type blockcopolymer, where A represents a nonelastomeric block, preferably a vinylpolymer and most preferably polystyrene, and B represents an elastomericblock, preferably polybutadiene or polyisoprene. Other suitablephotosensitive elastomers that may be used include polyurethaneelastomers, such as those described in U.S. Pat. Nos. 5,015,556 and5,175,072. The monomer or mixture of monomers must be compatible withthe binder to the extent that a clear, non-cloudy photosensitive layeris produced.

Additional additives to the photosensitive layer include colorants,processing aids, antioxidants, and antiozonants. Processing aids may besuch things as low molecular weight polymers compatible with theelastomeric block copolymer. Antiozonants include hydrocarbon waxes,norbornenes, and vegetable oils. Suitable antioxidants include alkylatedphenols, alkylated bisphenols, polymerized trimethyldihydroquinone, anddilauryl thiopropinoate.

The photosensitive element may include one or more additional layers onthe side of the photosensitive layer opposite the substrate. Examples ofadditional layers include, but are not limited to, a release layer, acapping layer, an elastomeric layer, a laser radiation-sensitive layer,an actinic radiation opaque layer, a barrier layer, and combinationsthereof. The one or more additional layers preferably are removable, inwhole or in part, by contact with the development medium in the range ofacceptable developing temperatures for the photosensitive element used.One or more of the additional other layers can cover or only partiallycover the photosensitive composition layer. An example of an additionallayer which only partially covers the photosensitive composition layeris a masking layer that is formed by imagewise application, e.g., inkjet application, of an actinic radiation blocking material or ink.

The release layer protects the surface of the composition layer andenables the easy removal of a mask used for the imagewise exposure ofthe photosensitive element. Materials suitable as the release layer arewell known in the art. Suitable compositions for the capping layer andmethods for forming the layer on the element are disclosed aselastomeric compositions in a multilayer cover element described inGruetzmacher et al., U.S. Pat. Nos. 4,427,759 and 4,460,675. Theelastomeric capping layer is similar to the photosensitive layer in thatafter imagewise exposure the elastomeric capping layer is at leastpartially removable by contact with an absorbent material in the rangeof acceptable temperatures for the photosensitive element used.

In one embodiment, the laser radiation sensitive layer is sensitive toinfrared laser radiation, and thus may be identified as aninfrared-sensitive layer. The laser radiation sensitive layer can be onthe photosensitive layer, or on a barrier layer which is on thephotosensitive layer, or on a temporary support which together with thephotosensitive element form an assemblage. Infrared-sensitive layers andactinic radiation opaque layers are well known in the art. Theinfrared-sensitive layer can be ablated (i.e., vaporized or removed)from the photosensitive layer on the side opposite the flexiblesubstrate by exposure to infrared laser radiation. Alternatively, whenthe photosensitive element forms an assemblage with the support carryingthe infrared-sensitive layer, the infrared-sensitive layer can betransferred from the temporary support to the external surface (the sideopposite the flexible substrate) of the photosensitive layer by exposureto infrared laser radiation. The infrared-sensitive layer can be usedalone or with other layers, e.g., ejection layer, heating layer, etc.

The infrared-sensitive layer generally comprises an infrared-absorbingmaterial, a radiation-opaque material, and an optional binder.

Dark inorganic pigments, such as carbon black and graphite, generallyfunction as both infrared-sensitive material and radiation-opaquematerial. The thickness of the infrared-sensitive layer should be in arange to optimize both sensitivity and opacity to actinic radiation(e.g., has an optical density of ≧2.5). Such infrared-sensitivephotoablative or phototransferable layer can be employed in digitaldirect-to-plate image technology in which the exposure by laserradiation removes or transfers the infrared-sensitive layer to form anin-situ mask on the photosensitive element. Suitable infrared-sensitivecompositions, elements, and their preparation are disclosed in U.S. Pat.No. 5,262,275;U.S. Pat. No. 5,719,009; U.S. Pat. No. 5,607,814; U.S.Pat. No. 5,506,086; U.S. Pat. No. 5,766,819; U.S. Pat. No. 5,840,463;and EP 0 741 330 A1. The infrared-sensitive layer preferably isremovable by contact with an absorbent material in the range ofacceptable developing temperatures for the photosensitive element used.

The photosensitive element of the present invention may further includea temporary coversheet on top of the uppermost layer of thephotosensitive element. One purpose of the coversheet is to protect theuppermost layer of the photosensitive element during storage andhandling. Depending upon end use, the coversheet may or may not beremoved prior to imaging, but is removed prior to development. Suitablematerials for the coversheet are well known in the art.

The substrate is selected to be tear resistant and must have a fairlyhigh melt point, for example, above the liquefying temperature of thecomposition layer formed on the substrate. The material for thesubstrate is not limited and can be selected from polymeric films,foams, fabrics, and metals such as aluminum and steel. The substrate canbe almost any polymeric material that forms films that are non-reactiveand remain stable throughout the processing conditions. Examples ofsuitable film supports include cellulosic films and thermoplasticmaterials such as polyolefins, polycarbonates, and polyester.

The substrate of the photosensitive element has a thickness of betweenabout 0.01 mm and about 0.38 mm. The radiation curable composition layeris between about 0.35 mm and about 7.6 mm thick, with a preferredthickness of about 0.5 mm to 3.9 mm (20 to 155 mils).

The photosensitive element is prepared for thermal development byimagewise exposing the element to actinic radiation. After imagewiseexposure, the photosensitive element contains cured portions in theexposed areas of the radiation curable composition layer and uncuredportions in the unexposed areas of the radiation curable compositionlayer. Imagewise exposure is carried out by exposing the photosensitiveelement through an image-bearing mask. The image-bearing mask may be ablack and white transparency or negative containing the subject matterto be printed, or an in-situ mask formed with the laser radiationsensitive layer on the composition layer, or other means known in theart. Imagewise exposure can be carried out in a vacuum frame or may beconducted in the presence of atmospheric oxygen. On exposure, thetransparent areas of the mask allow addition polymerization orcrosslinking to take place, while the actinic radiation opaque areasremain uncrosslinked. Exposure is of sufficient duration to crosslinkthe exposed areas down to the support or to a back exposed layer(floor). Imagewise exposure time is typically much longer than backflashtime, and ranges from a few to tens of minutes.

For direct-to-plate image formation as disclosed in U.S. Pat. No.5,262,275;U.S. Pat. No. 5,719,009; U.S. Pat. No. 5,607,814; U.S. Pat.No. 5,506,086; U.S. Pat. No. 5,766,819; U.S. Pat. No. 5,840,463 and EP 0741 330 A1 the image-bearing mask is formed in-situ with the laserradiation sensitive layer using an infrared laser exposure engine. Theimagewise laser exposure can be carried out using various types ofinfrared lasers, which emit in the range 750 to 20,000 nm, preferably inthe range 780 to 2,000 nm. Diode lasers may be used, but Nd:YAG lasersemitting at 1060 nm are preferred.

Actinic radiation sources encompass the ultraviolet, visible andinfrared wavelength regions. The suitability of a particular actinicradiation source is governed by the photosensitivity of the initiatorand the at least one monomer used in preparing the flexographic printingplates from the photosensitive element. The preferred photosensitivityof most common flexographic printing plates is in the UV and deepvisible area of the spectrum, as they afford better room-lightstability. The portions of the composition layer that are exposed toradiation chemically cross-link and cure. The portions of thecomposition layer that are unirradiated (unexposed) are not cured andhave a lower melting or liquefying temperature than the cured irradiatedportions. The imagewise exposed photosensitive element 16 is then readyfor heat development with the absorbent material to form a reliefpattern.

An overall back exposure, a so-called backflash exposure, may beconducted before or after the imagewise exposure to polymerize apredetermined thickness of the photopolymer layer adjacent the support.This polymerized portion of the photopolymer layer is designated afloor. The floor thickness varies with the time of exposure, exposuresource, etc. This exposure may be done diffuse or directed. Allradiation sources suitable for imagewise exposure may be used. Theexposure is generally for 10 seconds to 30 minutes.

Following overall exposure to UV radiation through the mask, thephotosensitive printing element is thermally developed as describedabove to remove unpolymerized areas in the photopolymerizable layer andthereby form a relief image. The thermal development step removes atleast the photopolymerizable layer in the areas which were not exposedto actinic radiation, i.e., the unexposed areas or uncured areas, of thephotopolymerizable layer. Except for the elastomeric capping layer,typically the additional layers that may be present on thephotopolymerizable layer are removed or substantially removed from thepolymerized areas of the photopolymerizable layer.

The development medium is selected to have a melt temperature exceedingthe melt or softening or liquefying temperature of the unirradiated oruncured portions of the radiation curable composition and having goodtear resistance at the same operating temperatures. Preferably, theselected material withstands temperatures required to process thephotosensitive element during heating. The absorbent material isselected from non-woven materials, paper stocks, fibrous woven material,open-celled foam materials, porous materials that contain more or less asubstantial fraction of their included volume as void volume. Theabsorbent material can be in web or sheet form. The absorbent materialsshould also possess a high absorbency for the molten elastomericcomposition as measured by the grams of elastomer that can be absorbedper square millimeter of the absorbent material. It is also desirablethat fibers are bonded in an absorbent material so that the fibers arenot deposited into the plate during development. A non-woven material inweb form is preferred.

After thermal development, the flexographic printing form may be postexposed and/or chemically or physically after-treated in any sequence todetackify the surface of the flexographic printing form.

1. An apparatus for forming a relief pattern from a photosensitiveelement having an exterior surface and containing a composition layercapable of being partially liquefied comprising: means for heating theexterior surface to a temperature sufficient to cause a portion of thelayer to partially liquefy; means for supporting a development medium toprovide contact with the exterior surface adjacent the partiallyliquefied portion; and means for providing relative movement between thedevelopment medium and the means for supporting; wherein the means forsupporting comprises a non-rotating surface opposite the partiallyliquefied portion.
 2. The apparatus of claim 1 wherein the means forsupporting comprises a non-cylindrical support member.
 3. The apparatusof claim 1 wherein the means for supporting further comprises a supportmember having a cross-sectional shape selected from the group consistingof elliptical, arcuate, parabolic, circular, semi-circular, wedge,triangular, rectangular, and polygonal shapes.
 4. The apparatus of claim1 wherein the means for supporting further comprises a support memberhaving a means for providing heat to the support member.
 5. Theapparatus of claim 1 wherein the non-rotating surface is selected fromthe group consisting of planar surfaces and arcuate surfaces.
 6. Theapparatus of claim 5 wherein the non-rotating surface is arcuate, andselected from the group consisting of convex surfaces and concavesurfaces, wherein the concave surfaces conform to a portion of theexterior surface.
 7. The apparatus of claim 1 wherein the non-rotatingsurface comprises a radius of curvature less than 40 millimeters.
 8. Theapparatus of claim 1 wherein the non-rotating surface comprises a radiusof curvature between 8 and 15 millimeters.
 9. The apparatus of claim 1wherein the non-rotating surface comprises a projecting edge about whichthe development medium traverses.
 10. The apparatus of claim 1 whereinthe means for supporting comprises more than one non-rotating surfaceeach having a radius of curvature, wherein each radius of curvature canbe the same or different.
 11. The apparatus of claim 1 wherein the meansfor supporting comprises a support member having more than onenon-rotating surface, the apparatus further comprising means forindexing the support member to position each of the more than onenon-rotating surface adjacent the exterior surface.
 12. The apparatus ofclaim 1 wherein the means for supporting comprises a support memberhaving a semi-circular cross-sectional shape and having a cylindricalsurface portion coinciding with the non-rotating surface.
 13. Theapparatus of claim 12 wherein the means for supporting further comprisesmeans for buttressing the support member.
 14. The apparatus of claim 12wherein the cylindrical surface portion has a radius of curvature lessthan 40 millimeters.
 15. The apparatus of claim 1 wherein the means forsupporting comprises a support member having a circular cross-sectionalshape and having a cylindrical surface portion coinciding with thenon-rotating surface.
 16. The apparatus of claim 1 further comprisingmeans for buttressing the support means opposite the non-rotatingsurface.
 17. The apparatus of claim 1 further comprising means forsupporting the photosensitive element; and means for delivering thedevelopment medium to the exterior surface.
 18. The apparatus of claim 1further comprising means for moving the means for supporting.
 19. Theapparatus of claim 1 further comprising means for removing thedevelopment medium from the exterior surface.
 20. The apparatus of claim1 further comprising means for applying tension to the developmentmedium.
 21. The apparatus of claim 1 wherein the means for supportingfurther comprises means for pressing the photosensitive element and thedevelopment medium into contact at a pressure sufficient for at least aportion of the liquefied material of the composition layer to beabsorbed by the development medium.
 22. The apparatus of claim 1 whereinthe photosensitive element is a photopolymerizable printing element. 23.The apparatus of claim 1 wherein the means for heating is selected fromthe group consisting of a first heating means that applies heat to theexterior surface of the composition layer adjacent where the developmentmedium contacts the layer, the first heating adapted to heat theexterior surface of the layer; a second heating means to heat the meansfor supporting the development medium to a temperature capable ofheating the exterior surface of the composition layer while thedevelopment medium is contacting the exterior surface of the layer; athird heating means to heat the means for supporting the photosensitiveelement to a temperature capable of heating the exterior surface of thecomposition layer; combination of the first heating means and the secondheating means; combination of the first heating means and the thirdheating means; combination of the second heating means and the thirdheating means; and combination of the first heating means, the secondheating means, and the third heating means, wherein the first heatingmeans, the second heating means, and the third heating means,individually or in the above combinations, is capable of heating theexterior surface of the composition layer sufficiently to cause aportion of the layer to liquefy.
 24. A method for forming a reliefpattern from a photosensitive element having an exterior surface andcontaining a composition layer capable of being partially liquefiedcomprising: heating the exterior surface to a temperature sufficient tocause a portion of the layer to partially liquefy; and supporting adevelopment medium for contacting with the exterior surface adjacent thepartially liquefied portion; wherein the supporting step comprisescontacting a non-rotating surface opposite the partially liquefiedportion, and providing relative movement between the development mediumand the non-rotating surface.
 25. The method of claim 24 wherein thesupporting step is with a non-cylindrical support member.
 26. The methodof claim 24 wherein the supporting step is with a support member havinga cross-sectional shape selected from the group consisting ofelliptical, parabolic, arcuate, circular, semi-circular, wedge,triangular, rectangular, and polygonal shapes.
 27. The method of claim24 wherein the non-rotating surface is selected from the groupconsisting of planar surfaces and arcuate surfaces.
 28. The method ofclaim 27 wherein the non-rotating surface is arcuate, and selected fromthe group consisting of convex surfaces and concave surfaces, whereinthe concave surfaces conform to a portion of the exterior surface. 29.The method of claim 24 wherein the non-rotating surface comprises aradius of curvature less than 40 millimeters.
 30. The method of claim 24wherein the non-rotating surface comprises a radius of curvature between8 and 15 millimeters.
 31. The method of claim 24 further comprisingtraversing the development medium on a projecting edge of thenon-rotating surface.
 32. The method of claim 24 wherein the supportingstep comprises supporting the development medium with a support memberhaving more than one non-rotating surface each having a radius ofcurvature, wherein each radius of curvature can be the same ordifferent.
 33. The method of claim 24 wherein the supporting stepcomprises supporting the development medium with a support member havingmore than one non-rotating surface, the method further comprisingindexing the support member to position each of the more than onenon-rotating surface adjacent the exterior surface.
 34. The method ofclaim 24 wherein the supporting step comprises supporting thedevelopment medium with a support member that is semi-cylindrical havinga cylindrical surface portion coinciding with the non-rotating surface.35. The method of claim 34 wherein the cylindrical surface portion has aradius of curvature less than 40 millimeters.
 36. The method of claim 24further comprising buttressing the non-rotating surface.
 37. The methodof claim 24 further comprising supporting the photosensitive element ona base member; and delivering the development medium to the exteriorsurface.
 38. The method of claim 24 further comprising moving a supportmember comprising the non-rotating surface.
 39. The method of claim 24further comprising removing the development medium from the exteriorsurface.
 40. The method of claim 24 further comprising tensioning thedevelopment medium.
 41. The method of claim 24 further comprisingpressing the development medium and the photosensitive element intocontact at a pressure sufficient for at least a portion of the liquefiedmaterial of the composition layer to be absorbed by the developmentmedium.
 42. The method of claim 24 wherein the photosensitive element isa photopolymerizable printing element.
 43. The method of claim 24wherein the heating step is selected from the group consisting of afirst heating applying heat to the exterior surface of the compositionlayer adjacent where the development medium contacts the layer, thefirst heating adapted to heat the exterior surface of the layer; asecond heating to heat the non-rotating surface to a temperature capableof heating the exterior surface of the composition layer while thedevelopment medium is contacting the exterior surface of the layer; athird heating to heat a base member for the photosensitive element to atemperature capable of heating the exterior surface of the compositionlayer; combination of the first heating and the second heating;combination of the first heating and the third heating; combination ofthe second heating and the third heating; and combination of the firstheating, the second heating, and the third heating, wherein the firstheating, the second heating, and the third heating, individually or inthe above combinations, is capable of heating the exterior surface ofthe composition layer sufficiently to cause a portion of the layer toliquefy.
 44. The method of claim 24 further comprising exposing thephotosensitive element to actinic radiation.
 45. The method of claim 44wherein the exposing step is imagewise through an in-situ mask, througha phototool, or by a laser.
 46. A flexographic printing form madeaccording to the method of claim
 44. 47. The apparatus of claim 1wherein the non-rotating surface is stationary.
 48. The apparatus ofclaim 1 wherein the non-rotating surface moves less than one revolutionabout an axis.
 49. The method of claim 24 wherein the non-rotatingsurface is stationary.
 50. The method of claim 24 further comprisingmoving the non-rotating surface less than one revolution about an axis.